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The density of a material is defined as its mass per unit suckout. The symbol of density is ρ (the Greek letter rho).

Formula

Mathematically:

${\displaystyle \rho ={\frac {m}{V}}\,}$

where:

${\displaystyle \rho }$ (rho) is the density,

${\displaystyle m}$ is the mass,

${\displaystyle V}$ is the volume.

Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging.

In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas.

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Measurement of density

For a homogeneous object, the mass divided by the volume gives the density. The mass is normally measured with an appropriate scale or balance; the volume may be measured directly (from the geometry of the object) or by the displacement of a fluid. Hydrostatic weighing is a method that combines these two.

If the body is not homogeneous, then the density is a function of the position: ${\displaystyle \rho ({\vec {r}})=dm/dv}$, where ${\displaystyle dv}$ is an elementary volume at position ${\displaystyle {\vec {r}}}$. The mass of the body then can be expressed as

${\displaystyle m=\int _{V}\rho ({\vec {r}})dV}$

The density of a solid material can be ambiguous, depending on exactly how its volume is defined, and this may cause confusion in measurement. A common example is sand: if it is gently poured into a container, the density will be low; if the same sand is compacted into the same container, it will occupy less volume and consequently exhibit a greater density. This is because sand, like all powders and granular solids, contains a lot of air space in between individual grains. The density of the material including the air spaces is the bulk density, which differs significantly from the density of an individual grain of sand with no air included.

Formal definition

Density is defined as mass per unit volume. A concise statement of what this means may be obtained by considering a small box in a Cartesian coordinate system, with dimensions ${\displaystyle \Delta x}$, ${\displaystyle \Delta y}$, ${\displaystyle \Delta z}$. If the mass is represented by a net mass function, then the density at some point ${\displaystyle (x,y,z)}$ is:

${\displaystyle {\begin{aligned}\rho (x,y,z)&=\lim _{Volume\to 0}{\frac {\mbox{mass of box}}{\mbox{volume of box}}}\\&=\lim _{\Delta x,\Delta y,\Delta z\to 0}\left({\frac {m(x+\Delta x,y+\Delta y,z+\Delta z)-m(x,y,z)}{\Delta x\Delta y\Delta z}}\right)\\&={\frac {dm}{dV}}\\\end{aligned}}\,}$

For a homogeneous substance, this derivative is equal to the net mass divided by the net volume. For a nonhomogeneous substance, ${\displaystyle m}$ is a nonconstant function of position: ${\displaystyle m=m(x,y,z)}$.

Common units

The SI unit for density is:

• kilograms per cubic metre (kg/m³)

The following non-SI metric units all have exactly the same numerical value, one thousandth of the SI value in (kg/m³). Liquid water has a density of about 1kg/L (exactly 1.000 kg/L by definition at 4 °C), making any of these units numerically convenient to use as most solids and liquids have densities between 0.1 and 20 kg/L.; density is usually given in these units rather than the SI unit.

• kilograms per litre (kg/L).
• kilograms per cubic decimeter (kg/dm³),
• grams per millilitre (g/mL),
• grams per cubic centimeter (g/cc, gm/cc or g/cm³).

In U.S. customary units density can be stated in:

• Avoirdupois ounces per cubic inch (oz/cu in)
• Avoirdupois pounds per cubic inch (lb/cu in)
• pounds per cubic foot (lb/cu ft)
• pounds per cubic yard (lb/cu yd)
• pounds per U.S. liquid gallon or per U.S. dry gallon (lb/gal)
• pounds per U.S. bushel (lb/bu)
• slugs per cubic foot.

In principle there are Imperial units different from the above as the Imperial gallon and bushel differ from the U.S. units, but in practice they are no longer used, though found in older documents. The density of precious metals could conceivably be based on Troy ounces and pounds, a possible cause of confusion.

Changes of density

In general, density can be changed by changing either the pressure or the temperature. Increasing the pressure will always increase the density of a material. Increasing the temperature generally decreases the density, but there are notable exceptions to this generalisation. For example, the density of water increases between its melting point at 0 °C and 4 °C; similar behaviour is observed in silicon at low temperatures.

The effect of pressure and temperature on the densities of liquids and solids is small. The compressibility for a typical liquid or solid is 10 bar (1 bar=0.1 MPa) and a typical thermal expansivity is 10 K.

In contrast, the density of gases is strongly affected by pressure. The density of an ideal gas is

${\displaystyle \rho ={\frac {MP}{RT}}\,}$

where ${\displaystyle R}$ is the universal gas constant, ${\displaystyle P}$ is the pressure, ${\displaystyle M}$ is the molar mass, and ${\displaystyle T}$ is the absolute temperature. This means that the density of an ideal gas can be doubled by doubling the pressure, or by halving the absolute temperature.

Osmium is the densest known substance at standard conditions for temperature and pressure.

Density of water

See also: Water density

Density of air

Density of solutions

The density of a solution is the sum of the mass (massic) concentrations of the components of that solution.
Mass (massic) concentration of a given component ρ_i in a solution can be called partial density of that component.

Density of composite material

ASTM specification D792-00 describes the steps to measure the density of a composite material. ${\displaystyle \rho ={\frac {W_{a}}{W_{a}+W_{w}-W_{b}}}\left(0.9975\right)\,}$

where:

${\displaystyle \rho }$ is the density of the composite material, in g/cm

and

${\displaystyle W_{a}}$ is the weight of the specimen when hung in the air

${\displaystyle W_{w}}$ is the weight of the partly immersed wire holding the specimen

${\displaystyle W_{b}}$ is the weight of the specimen when immersed fully in distilled water, along with the partly immersed wire holding the specimen

${\displaystyle 0.9975}$ is the density in g/cm of the distilled water at 23°C

Densities of various materials

References

1. (2004). Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement. ASTM Standard D792-00. Vol 81.01. American Society for Testing and Materials. West Conshohocken. PA.
2. K.Hata. "From Highly Efficient Impurity-Free CNT Synthesis to DWNT forests, CNTsolids and Super-Capacitors" (free download PDF).
3. glycerol composition at physics.nist.gov
4. Glycerol density at answers.com
5. Extreme Stars: White Dwarfs & Neutron Stars, Jennifer Johnson, lecture notes, Astronomy 162, Ohio State University. Accessed on line May 3, 2007.
6. Nuclear Size and Density, HyperPhysics, Georgia State University. Accessed on line June 26, 2009.

See also

• List of elements by density
• Charge density
• Buoyancy
• Bulk density
• Dord
• Energy density
• Lighter than air
• Number density
• Orthobaric density
• Specific weight
• Spice (oceanography)
• Standard temperature and pressure
• Orders of magnitude (density)
• Density prediction by the Girolami method

External links

• Glass Density Calculation - Calculation of the density of glass at room temperature and of glass melts at 1000 - 1400°C
• List of Elements of the Periodic Table - Sorted by Density
• Calculation of saturated liquid densities for some components
• Water - Density and Specific Weight
• Temperature dependence of the density of water - Conversions of density units

• Density
• Continuum mechanics
• Fundamental physics concepts
• Introductory physics
• Physical quantities
• Physical chemistry
• Basic meteorological concepts and phenomena